Update Sphinx, Theming, and Faster Builds (#1354)

* Fix parallel Sphinx builds when building man output
* Don't use vendored theme
* Remove vendored theming
* Update SphinxBuild.cmake for newer CMake and to use parallel builds
* Update Sphinx version
* Fix: Documentation version was garbled by debug output
* Use the dirhtml builder:

Generates "prettier" URLs. To support the old URLs, also generate HTML
files that cause a redirect to the new URLs.

* Use the "furo" theme.